Looking-glass

It is unlucky to break a looking-glass. The nature of the
ill-luck varies; thus, if a maiden, she will never marry; if a married
woman, it betokens a death, etc. This superstition arose from the use
made of mirrors in former times by magicians. If in their operations
the mirror used was broken, the magician was obliged to give over his
operation, and the unlucky inquirer could receive no answer.

Looking-glass of Lao

reflected the mind as well as the outward form. (Citizen of the
World, xlv.)